Elevated antibody titers to the viral capsid antigens of Epstein-Barr virus (EBV) over a level of 1:60 will be used as a possible epidemiological marker of an impaired immune response to primary EBV infections. Antibody tests will be carried out on patients convalescent from infectious mononucleosis, on healthy adults, and on patients with Hodgkin's disease to identify such persons. More intensive studies of humoral and cell-mediated immunity will then be made in an effort to demonstrate the existence and nature of an impaired immune response.